Trust pervades human societies1, 2. Trust is indispensable in friendship, love, families and organizations, and plays a key role in economic exchange and politics3. In the absence of trust among trading partners, market transactions break down. In the absence of trust in a country’s institutions and leaders, political legitimacy breaks down. Much recent evidence indicates that trust contributes to economic, political and social success4, 5. Little is known, however, about the biological basis of trust among humans. Here we show that intranasal administration of oxytocin, a neuropeptide that plays a key role in social attachment and affiliation in non-human mammals6, 7, 8, causes a substantial increase in trust among humans, thereby greatly increasing the benefits from social interactions. We also show that the effect of oxytocin on trust is not due to a general increase in the readiness to bear risks. On the contrary, oxytocin specifically affects an individual’s willingness to accept social risks arising through interpersonal interactions. These results concur with animal research suggesting an essential role for oxytocin as a biological basis of prosocial approach behaviour.